A Wollongong woman who used a gun to rob a convenience store later claimed to police the weapon was a toy replica, a court has heard.

Melissa Louise Johnson pleaded guilty in Wollongong Local Court on Wednesday to a single charge of robbery armed with a dangerous weapon after confessing to holding up the Welcome Mart convenience store at Koonawarra on October 20 last year.

Police documents outlining Johnson's crime say the 32-year-old walked into the store just after 7pm and asked for milk and cigarettes.

While being served by a male staff member, Johnson lifted up her shirt to reveal a black pistol tucked into the top of her pants. She pointed to it and said to the man: "And the money. All the money in the till."

The man removed about $3000 in cash and handed it to Johnson, who stashed it in her bag and left the store.

The man called to another staff member to contact police before following Johnson outside.

He told police he saw her get into the passenger seat of a white hatchback car, which drove off towards Kanahooka.

Officers who attended the scene issued a description of Johnson to the media later the same day.

The following morning, police received a call from an anonymous person who said Johnson had come to their place straight after she had committed the crime, and had appeared "nervous and was sweating profusely".

Johnson was arrested that afternoon but denied any knowledge of, or involvement in the robbery when questioned by police.

However, the court heard at a later date she admitted the robbery, but claimed her co-accused, the driver of the car, was the instigator and "told her to do it".